Engine knock, which can create uncontrolled combustion in vehicle internal combustion engines, is a condition that typically occurs when ignition timing of the vehicle engine is advanced improperly. To avoid engine knock, which can lead to engine damage, engine knock sensors are often used. Engine knock sensors are typically configured to detect which cylinder or cylinders of an internal combustion engine are experiencing a knock condition. When engine knock sensors are coupled to vehicle engine control modules that control the operation of the engine, the vehicle engine control module can monitor the engine knock sensors, and modify the ignition timing of the engine until the knock condition is no longer detected by the engine knock sensors. Engine knock sensors can also be employed to help vehicle engine control modules to know how to adjust the timing of the engine to provide improved fuel economy and torque.
To insure that engine knock sensors are operating properly, it is typically required to monitor the engine knock sensors periodically. More specifically, it can be necessary to diagnose situations in which the engine knock sensors are in an open circuit condition, short circuit condition, shorted to the battery voltage of the vehicle, and/or shorted to ground. In order to diagnose and remedy these sensor fault conditions, the engine knock sensors are typically monitored while the engine is running.
In one conventional method used to detect open and short circuit conditions in engine knock sensors, the values of the output signal of the engine knock sensor are accumulated over a relatively long period of time, and analyzed by a processor. The processor uses algorithms to determine the noise floor of the sensor based on the gathered engine knock sensor output values. The processor further processes this information to determine if the sensor is in an open circuit, short circuit, or normal condition.
In another conventional method, a dedicated analog-to-digital port and external circuit are used to sense the input common mode voltage of the sensor and compare it to some pre-defined threshold. If the common mode voltage shifts relative to the pre-defined threshold, a short circuit condition in the sensor may be indicated. However, this method is typically unable to distinguish between a short to battery and a short to ground condition. In addition, this method is typically unable to distinguish between a short across the sensor and an open sensor condition.
In still another conventional method, an AC test signal is injected into the sensor, rectified, and compared to a threshold value to determine whether the sensor is in an open circuit condition, short circuit condition, or normal operating condition. While typically providing better results than the first two methods discussed above, an accurate AC test signal typically needs to be generated on the chip. This can be difficult to implement in some integrated circuit design processes, and thus, complicates the circuit design and cost. In addition, this method generally does not provide for a distinction between short circuit to battery conditions and short circuit to ground conditions.
In yet another conventional method, comparators are integrated into the sensor interface circuitry to measure the voltage at the positive and negative terminals of the sensor to discriminate between short to battery conditions and short to ground conditions. However, this method is typically unable to differentiate between short circuit conditions across the sensor, open circuit conditions, and conditions in which the sensor is operating normally. In addition, this method generally is less desirable due to the fact that two comparators are typically required at each of the positive and negative terminals of the sensor, adding to overall system cost.
What is needed is a cost-effective method and sensing circuitry for identifying open circuit, short circuit, and normal operating conditions in engine knock sensors.